Typically, in hard disk drives, circular data tracks are formed concentrically on a magnetic disk. Information is read from or written to the disk after a seek operation is performed. That is, the disk is rotated and a magnetic head is moved radially over the disk to position the head over a desired data track. To position the head over the desired data track, the head reads prerecorded head position identification information and burst patterns from the disk.
FIG. 1 shows an exemplary disk suitable for practicing the present invention. Each track on the disk 18 is divided into data regions 52 and servo regions 50. The head recognizes the position on the disk 18 based on servo data recorded in the servo region 50. After positioning the head over the desired data track, data is read from or written to the disk 18.
The servo data comprises head position identification information including cylinder (CYL) and sector (SEC) position information which are recorded with a gray code and a binary code respectively. A burst pattern is also recorded in the servo data following the gray and binary codes. The burst pattern provides a signal, i.e., a fine-servo signal, which is used to make fine adjustments to position the head over the desired track. The head is positioned over the track by reading the blocks of servo data.
FIG. 2 shows a portion of the head position identification information and burst pattern regions recorded on the disk suitable for practicing the present invention. In FIG. 2, the disk rotates in the circumferential direction as indicated by arrow F. The head (not shown) moves radially across the disk as indicated by an arrow G. The disk has a plurality of concentric circular data tracks 100A, 100B, 100C, . . . where blocks of data are recorded. The head position identification information is recorded in a head position identification region 102 and the burst pattern is recorded in a burst pattern region 104. The head position identification information region 102 and burst pattern region 104 are located between adjacent data tracks.
To identify a particular data track, each data track is given a track address. A predetermined number of bits of identification information are recorded in the head position identification region 102. The cylinder address of the corresponding data track is encoded with a gray code, which is a cyclic binary code. The sector address is encoded with a binary code. A plurality of burst pattern strings 106A, 106B, 106C, and 106D are recorded in the burst pattern region 104. For example, in the shaded area of FIG. 4, four burst pattern strings 106A, 106B, 106C, and 106D are arranged radially along the disk.
The head outputs a read signal each time the head is disposed over the head position identification information region 102. To position the head with respect to the desired data track, the track address of the data track below the head is calculated based on the head position identification information contained in the read signal. When the head reaches the desired data track, signals obtained by reading the plurality of burst pattern strings are analyzed, and a position detection signal is generated. The amplitude of the position detection signal changes linearly depending on the position of the head with respect to the burst pattern signals. Based on the position detection signal, the center of the gap of the head is positioned over the centerline of the target data track.
In a disk drive having multiple heads, the heads are switched when sequential data is written to or read from a cylinder, which comprises different disk surfaces of the disk drive. For example, if a disk drive has two disks and four heads, numbered 0, 1, 2, and 3, then the heads are typically switched in the sequence of the head numbers 0, 1, 2, and 3 during a data read or write operation. That is, if a data write operation using head 0 reaches the end of track 2500 on the disk, then head 0 is switched to head 1 and the data write operation continues in the same cylinder at track number 2500 corresponding to head 1. Similarly, sequential data continues to be written to cylinder number 2500 using heads 2 and 3.
In disk drives having multiple disks, the heads are arranged in a fixed vertical or radial relationship with respect to one another. FIG. 3 shows the relative position between heads when the servo information is recorded on the disk. Generally, servo information on all disk surfaces is radially aligned. As a consequence, a seek operation following a head switch is not necessary. In other words, as shown in FIG. 3, when one head, 0, 1, 2 or 3, is positioned opposite a desired track n, the other heads are also positioned opposite the desired track n in the same cylinder.
However, when assembling heads and disks having the prerecorded servo information, a relative offset occurs between the heads radially along the disk, i.e. in the off-track direction. The radial or off-track offset is caused for example, by the inclination of the rotary shaft of the spindle motor, the inclination of the VCM for a head, or the distortion from clamping. More specifically, when writing servo data with a servo writer, the servo pattern is written at a position having no disk shift. However, distortion occurs when the servo writer is removed during the process to remove and attach the clamp, and in subsequent disk drive operation over time. Therefore, as shown in FIG. 4, the heads have a relative radial or off-track offset .beta. in the assembled disk drive.
Assume that the aforementioned head switching operation is executed in a disk drive having heads positioned as shown in FIG. 4. Although FIG. 4 shows each head having a circumferential offset .alpha. with respect to the other heads, i.e, offset along the circumference of the disk, for easier understanding, in practice there is no circumferential offset between heads. When data is written to track n with head 0 and then to track n with head 1, head 1 is moved from its position opposite track n+1 to track n. Therefore, a one track seek operation is required. Therefore, if a disk drive has an off-track offset between heads, a seek operation becomes necessary to properly align the active head when heads are switched. If the amount of off-track offset increases, then, after switching heads, the distance to move the head to the desired track increases. Because the seek distance increases, the seek time also increases. When reading or writing a long stream of data, many seek operations need to be executed. Therefore, reduction of seek time is an important factor in improving disk drive performance.